1. Field of the Invention
The present invention relates to the art of thermal transfer printing of a type designed to effect a printing with the use of a thermal print head for transferring a thermally transferable ink from an ink carrier ribbon onto a recording medium such as, for example, paper.
2. Description of the Prior Art
With the advent of the age of widespread use of computers, particularly personal computers, the use of electrothermal matrix printers or dot matrix printers is currently increasing as a computer output device that provides a hard copy of a text. Of these electrothermal matrix printers, an electrothermal transfer printer is also known as a non-impact matrix printer and utilizes a thermal print head operable to transfer a thermally fusible ink from an ink carrier ribbon onto a recording medium while the thermal print head moves along a line being printed. The electrothermal transfer printer can also be operable without the ink carrier ribbon if the recording paper used in connection therewith is a heat sensitive paper which exhibits a color rendering capability when heated.
The conventional electrothermal transfer printer and the printing method performed thereby, both being disclosed in, for example, the Japanese Laid-open Patent Publication No. 56-123877, published in 1981, will now be discussed with reference to FIGS. 8 to 10 of the accompanying drawings.
The electrothermal transfer printer particularly shown in FIG. 8 comprises a cylindrical platen 1 supported for intermittent rotation about its own longitudinal axis and drivingly coupled with a drive motor as is well known to those skilled in the art, a recording paper 2 being transported around the platen 1 in synchronism with the intermittent rotation of the platen 1. The thermal printer also comprises a thermal print head 3 mounted on a carriage 4 for movement together therewith in a direction parallel to the longitudinal axis of the platen 1 and also for movement between operative and inoperative positions in a direction both towards and away from the platen 1, respectively, and a carriage transport mechanism generally identified by 10. The carriage 4 has a pair of spindles for the support thereon of respective reels to which opposite ends of a length of ink carrier ribbon 7 are secured, a substantially intermediate portion of the length of ink carrier ribbon 7 being wound around one or both of the reels 5 and 6. In practice, the spindles for the support thereon of the respective reels 5 and 6 are operatively associated with the movement of the carriage 4 and adapted to be alternately brought into engagement with a ribbon drive mechanism during the movement of the carriage 4 so that, when the spindle for the support thereon of the reel 5 is brought into engagement with the ribbon drive mechanism, the length of the ink carrier ribbon 7 can be transported from the reel 6 onto the reel 5 in a direction shown by the arrow X, but when the spindle for the support thereon of the reel 6 is brought into engagement with the ribbon drive mechanism, the length of the ink carrier ribbon 7 can be transported from the reel 5 onto the reel 6 in a direction shown by the arrow Y.
The ink carrier ribbon 7 comprises, as best shown in FIG. 9, a length of heat resistant base film 8 having its opposite ends connected with the respective reels 5 and 6 through associated leaders, and a thermally transferable ink layer 9 deposited over the entire length of the base film 8 on one of the opposite surfaces of the base film 8 which, when in use, confronts the platen 1.
Although not shown, a portion of the length of ink carrier ribbon 7 extending between the spaced reels 5 and 6 are, or may be, guided by one or more guide rollers rotatably mounted on the carriage 4.
The carriage transport mechanism 10 includes driven and drive pulleys 12 and 13 rotatably mounted on a base of a printer framework and spaced a distance enough to permit the thermal print head 3 to move from one end to the opposite end of the platen 1 together with the carriage 4, and an endless drive belt 11 trained around the driven and drive pulleys 12 and 13, one of the opposite runs of the endless drive belt 11 being fixed to the carriage 4 so that the movement of the endless drive belt 11 can result in the corresponding movement of the carriage 4. The drive pulley 13 is drivingly coupled with an electric reversible motor 14 so that the carriage 4 can be reciprocatingly moved between start and end positions in one of the opposite directions shown by the respective arrows a and b.
The thermal printer further comprises a controller 15 utilized to control respective coordinated movements of the electric reversible motor 14, the carriage 4 and the thermal print head 3.
The prior art thermal printer of the above described construction is so designed as to accomplish an actual printing of a particular alphanumeric character in the following manner. It is, however, to be noted that the thermal print head 3 has a heating element, identified by 3a in FIG. 9, with dot electrodes embedded therein, which dot electrodes are generally arranged in a vertical row and are adapted to be selectively energized electrically to heat a portion of the heating element 3a in a predetermined pattern corresponding to dots or indicia forming parts of a particular character desired to be printed.
When the actual printing is to take place, the thermal print head 3 is brought to a printing position where a particular alphanumeric character is to be printed on the recording paper 2 and is then moved from the inoperative position to the operative position at which the thermal print head 3 presses a portion of the ink carrier ribbon 7 against the recording paper 2 with the thermally transferable ink layer 9 held in tight contact with the recording paper 2. Prior to the movement of the thermal print head 3 from the inoperative position towards the operative position, the carriage 4 may, or may not, be moved in the direction shown by the arrow a in FIG. 8 depending on where the actual printing on the recording paper 2 begins.
No sooner is the thermal print head 3 brought to the operative position, than the heating element 3a shown in FIG. 9 is electrically heated according to an electric signal applied through the printer controller 15 and indicative of one or more dots forming parts of the particular alphanumeric character desired to be printed. As a result of the heating of the heating element 3a, that portion of the ink layer 9 on the ink carrier ribbon 7 which is then aligned with the heating element 3a is fused and transferred onto the recording paper 2. As the thermal print head 3 is moved away from the printing position together with the carriage 4 then moving in the direction parallel to the platen 1 with the length of ink carrier ribbon 7 separating progressively away from the recording paper 2, that portion of the ink layer 9 which has been so transferred onto the recording paper 2 forms, upon solidification, an ink deposit permanently fixed on the recording paper 2, as indicated by 9a in FIG. 9, while leaving a consumed area 9b on the base film 8 also as shown in FIG. 9. An aggregation of ink deposits 9a so formed may be an image of, or a part of the image of, the particular alphanumeric character.
The above described printing takes place while the carriage 4 is moved in the direction of the arrow a and, at the same time, the length of ink carrier ribbon 7 is transported from the reel 6 towards the reel 5 in a direction shown by the arrow X in FIG. 8. When a line of alphanumeric characters have been printed with the carriage 4 consequently brought to a right-hand position, the drive motor 14 is reversed to effect a return of the carriage 4 back to a left-hand position with the endless drive belt 11 moved in a direction shown by the arrow b. During the return movement of the carriage 4, the thermal print head 3 is moved from the operative position back to the inoperative position with the length of ink carrier ribbon 7 being kept away from the recording paper 2 and, at the same time, the transportation of the length of ink carrier ribbon 7 from the reel 6 towards the reel 5 is interrupted.
As hereinbefore described, the actual printing is carried out by heating the heating element 3a to permit that portion of the ink layer 9 to be thermally transferred from the base film 8 onto the recording paper 2 while that portion of the length of ink carrier ribbon 7 is pressed by the thermal print head 3 in the operative position against the recording paper 2 with the ink layer 9 held in contact therewith. During this actual printing, the heating element 3a of the thermal print head 3 is heated, in response to a drive signal fed from a drive circuit, to generate heat in a pattern corresponding to the image of the particular alphanumeric character with the resultant Joule heat transmitted to the ink layer 9 through the heat resistant base film 8 to fuse and then transfer that portion of the ink layer 9 onto the recording paper 2, that portion of the ink layer 9 so transferred being subsequently solidified to form the ink deposit 9a on the recording paper 2.
According to the prior art printing method as hereinbefore discussed, since that portion of the ink layer 9 fused by the heating element 3a is, before it is completely transferred onto the recording paper 2 to form the ink deposit 9a, separated from the interface of the base film 8 as a lump of ink material, that is, the interface separation takes place between the ink layer 9 and the base film 8, the consumed area 9b which has been left on the base film 8 as a result of the thermal transfer of the ink layer 9 onto the recording paper 2 in the manner as hereinabove described can no longer be used for the subsequent printing. Moreover, when the ink carrier ribbon 7 has been completely used from one end to the opposite end, the length of base film 8 still may have unconsumed fragments of the ink layer 9 each bound between the neighboring consumed area 9b as shown by 9c in FIG. 9, which unconsumed fragments of the ink layer 9 are unnecessarily discarded.
In any event, the interface separation referred to above will now be discussed in detail with particular reference to FIGS. 10a and 10b which illustrate exaggeratedly how that portion of the ink layer 9 is transferred onto the recording paper 2. In these figures, .tau. II represents the cohesive force acting in the ink layer 9 to hold ink particles together, .tau. IB represents the adhesive force acting between the ink layer 9 and the base film 8, and .tau. IP represents the adhesive force acting between the ink layer 9 and the surface of the recording paper 2.
Referring first to FIG. 10a, in the case where the recording paper 2 is of a type having a surface smoothness of 100 seconds as measured according to the Japanese Industrial Standards (JIS), which paper may be regarded as having a substantially specular surface, the ink deposit 9a formed by the portion of the ink layer 9 having been transferred onto the recording paper 2 as a result of the heating of the heating element 3a of the thermal print head 3 substantially completely sticks to the surface of the recording paper 2. Accordingly, at the time of separation of that portion of the ink layer 9 from the base film 8, the following relationships are simultaneously established, and that portion of the ink layer 9 is accordingly completely released from the base film 8 to accomplish the interface separation. EQU .tau. IP&gt;.tau. IB and .tau. II&gt;.tau. IB
In particular, the relationship of .tau. II&gt;.tau. IB is attributable to the fact that the ink deposit 9a on the recording paper 2 is fixed thereto as a result of that transferred portion of the ink layer 9 having been solidified. Also, while that transferred portion of the ink layer 9 is firmly retained in position on the surface 2a of the recording paper 2 by means of, for example, a well known capillary phenomenon, the ink layer 9 on the base film 8 is not retained by the base film 8 so firmly as the ink deposit 9a on the recording paper 2 and, therefore, the relationship of .tau. IP&gt;.tau. IB is in fact established.
Since unconsumed fragments 9c of the ink layer 9 each bound between the neighboring consumed area 9b on the base film 8 can no longer be used for the subsequent cycle of printing, the reeled ink carrier ribbon is unnecessarily disposed of.
In any event, that portion of the ink layer 9 which eventually forms the ink deposit 9a on the recording paper 2 has been more or less firmly interconnected with the film base 8 although it is not so hard as the ink deposit 9a interconnected with the recording paper 2. Accordingly, during a high speed printing with the carriage 4 traversing the recording paper 2 at a relatively high speed, the timing at which the ink carrier ribbon 7 being transported is separated away from the recording paper 2 turned around the platen 1 tends to be delayed with the point of separation occurring at a location spaced a distance from the actual printing position on the trailing side with respect to the direction of movement of the carriage 4 as shown in FIG. 11. Therefore, the length of ink carrier ribbon 7 tends to be slackened with the take-up reel consequently failing to wind up the length of ink carrier ribbon 7 properly. In view of this, such a high speed printing at a rate of, for example, 2 to 12 msec. per dot cannot be performed without difficulty.
On the other hand, in the case of the recording paper 2 having a surface smoothness of a few seconds according to JIS, which paper has a surface 2a with surface irregularities such as exaggeratedly shown in FIG. 10b, it has been found difficult to accomplish a uniform sticking of that thermally transferred portion of the ink layer 9 on the surface 2a of the recording paper 2. This is particularly true during the high speed printing because, even though the ink layer 9 is brought in contact with the surface 2a of the recording paper 2 as pressed by the thermal print head 3 through the base film 8, that portion of the ink layer 9 thermally separated from the base film 8 and being transferred onto the recording paper 2 tends to be quickly cooled to solidify before it penetrates sufficiently into some irregularities on the surface 2a of the recording paper 2. Specifically, in the case of the recording paper 2 having the rough surface as shown in FIG. 10b, that portion of the ink layer 9 transferred from the base film 8 deposits and subsequently solidifies on hill portions 2a1 forming parts of the surface irregularities on the surface 2a of the recording paper 2 without penetrating into dale portions 2a 2 also forming parts of the same surface irregularities.
The above described phenomenon is discussed in detail in a paper entitled "A BASIC STUDY OF THERMAL TRANSFER PRINTING FOR IMPROVEMENT OF PRINT QUALITY" written by S. Ando et al. of Rohm Co., Ltd. of Japan, which paper was made available to the assignee of the present invention about March, 1985.
In view of the foregoing, at the time of separation of that portion of the ink layer 9 from the base film 8, the following relationships can be simultaneously observed at the hill portions 2a1 of the rough surface 2a of the recording paper 2 and, accordingly, fragments 9aP of the ink layer 9 are released from the base film 8 to accomplish the interface separation. EQU .tau. IP&gt;.tau. IB and .tau. II&gt;.tau. IB
On the other hand, at the dale portions 2a2 of the rough surface of the same recording paper 2, the following relationships can be simultaneously observed: EQU .tau. IP&lt;.tau. IB and .tau. II&gt;.tau. IB
and the relationships of .tau. IP&gt;.tau. IB and .tau. II&gt;.tau. IB are no longer observed, with the consequence that fragments 9aB of that portion of the ink layer 9 remain on the base film 8 without being released therefrom. Once this happens, as shown in FIG. 10c, the printing quality tends to be deteriorated in such a way that the printed dots partially drop out and/or lacks the clarity, and, therefore, with the prior art printing method, the printing speed is limited and/or a high quality printing cannot be accomplished with papers other than the paper having a surface smoothness of about 100 seconds as measured according to JIS.
The prior art thermal transfer printing involves additional problems. By way of example, the outermost surface of the ink deposit 9a formed by the portion of the ink layer 9 having been thermally transferred onto the recording paper 2 is very smooth as if it were mirror-polished, in complementary relationship with the surface of the base film 8 to which it had adhered. Accordingly, when a reader reads a text full of characters delineated by the smooth ink deposits, the smooth ink deposits on the paper altogether reflect rays of light impinging thereupon, tending to accelerate fatique in the eyes of a reader.
Also, after the length of ink carrier ribbon 7 has been used for printing with the ink layer 9 having been transferred onto the recording paper at plural locations lengthwise of the ink carrier ribbon 7, it is usual that the base film 8 bears negative imprints (such as markings of the ink deposits 9a left on the base film 8) of characters which are complementary with the characters printed on the recording paper as positive imprints. Accordingly, the ink carrier ribbon now commerically available is not desirable in terms of the security of information and/or utmost care is required to dispose of the used ink carrier ribbon lest the printed information should be revealed to unauthorized persons.
In order to attain, inter alia, a high quality printing, some improvements have hitherto been made such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 60-23079, published Feb. 5, 1985, and the Japanese Laid-open Patent Publication No. 60-25781, published Feb. 8, 1985.
More specifically, the first mentioned patent publication discloses a method of and a means for allowing the length of ink carrier ribbon to be separated away from the recording paper before the portion of the ink layer having been thermally transferred by the thermal print head onto the recording paper to form the ink deposit is completely solidified. The improvement disclosed in this patent publication is directed to the predecessor wherein the length of ink carrier ribbon has been turned a substantial angular distance around the platen with the recording paper intervening between the angularly curved portion of the ink carrier ribbon and a corresponding portion of the platen.
One method disclosed in this patent publication is to allow the length of ink carrier ribbon to separate away from the recording paper in a direction generally tangential to the circumference of the platen immediately after the thermal transfer of that portion of the ink layer has taken place. The alternative method disclosed therein is the use of a post-heating means disposed on a leading side of the thermal print head with respect to the direction of movement of the length of ink carrier ribbon being taken up, which post-heating means is incorporated in the predecessor of the thermal printer of this patent publication for keeping the thermally transferred portions of the ink layer in a substantially softened state until the length of ink carrier ribbon being taken up is brought to a point of separation from the recording paper.
The purpose of the first mentioned patent publication is obviously to compensate for the difference in a coefficient of thermal expansion between the ink deposits and the recording paper which would constitute a cause of distorted prints particularly where the recording paper has a very smooth surface.
The second mentioned patent publication is directed to minimize the problem associated with the glary print finish on the recording paper. For this purpose, this second mentioned patent publication discloses three different methods. One of these methods is to use the increased temperature of head to be applied to the length of ink carrier ribbon during the actual printing so that that portion of the ink carrier which is applied with heat from the thermal print head for the transfer thereof onto the recording paper can be released partly, not completely, from the base film, the remaining portion being left adhering to the base film.
Another method disclosed in the second mentioned patent publication is to accelerate the separation of the length of ink carrier ribbon away from the recording paper, which takes place subsequent to the actual printing, so that the transfer of that portion of the ink layer can take place in a manner similar to that according to the first mentioned method. The remaining method disclosed in the second mentioned patent publication is to employ as small as possible the angle of separation at which the length of ink carrier ribbon separates away from the recording paper subsequent to the actual printing for the same purpose.
However, a series of experiments conducted by the inventors of the present invention with due regards paid to the disclosure made in any one of the above discussed patent publications have revealed that, even where the length of ink carrier ribbon is allowed to separate away from the recording paper subsequent to the actual printing while the ink deposit remains not completely solidified or in a substantially softened state, the interface separation of that portion of the ink layer from the base film and/or the lack of clarity of the ink deposits or insufficient print quality such as discussed with reference to FIGS. 10a and 10b occur, posing problems similar to those occurring when the length of ink carrier ribbon is separated away from the recording paper after the ink deposit formed by the transfer of that portion of the ink layer onto the recording paper has been solidified.